Although air pollution in China's capital city (see a related story) is almost always worse than anywhere in the United States, Chinese efforts to clean up the air before the Games have paid off.

The country shut down all nearby factories and ordered half the cars off the road, creating tangible improvements, scientists say.

"I'm measuring about a 20 to 40 percent reduction in particulate matter compared to a year ago," said Staci Simonich, an environmental chemist from Oregon State University whose lab group has made three trips to Beijing to study the city's air.

Ups and Downs

Day-to-day pollution levels have tended to fluctuate. When it rains, pollution drops, then builds back up—unless there's a strong north wind to blow it away, Simonich said.

"We've seen some real ups and downs," Simonich said from her temporary lab at Peking University in Beijing.

On a good day, the air quality is still below levels American athletes are accustomed to.

U.S. runner Yoder Begley's 10,000-meter teammate Kara Goucher grew up in the clean air of northern Minnesota, and had never been to Beijing before this summer.

"[T]he pollution and smog in Beijing is much, much worse than I imagined," she wrote earlier in the week—before the latest rain—on a blog for the Duluth News Tribune.

"Its a bit eerie how the sun never comes out all day. If you are walking around the village and you look ahead, you can't see all of the buildings. The pollution creates a fog that clouds over everything. It is unimaginable. I am shocked by how bad it is."